This invention relates to a hollow optical fiber and a method for manufacturing the same.
Attention is paid to hollow optical fibers as optical fibers for transmitting an infrared ray having a wavelength of from 2 xcexcm to ten-odd xcexcm which is useful in the medical and industrial field. Optical fibers of this type each have a core which is hollow and a clad formed of a metal layer such as silver or the like. A dielectric layer is disposed on an inner periphery of the metal layer for the purpose of enhancing a transmission rate. As one example of the material of a dielectric which has been put to practical use, there is silver iodide. As those for which search and development are undergoing, there are germanium and cyclic olefin polymer. In the dielectric materials listed above, however, there exists an absorption peak P where absorption of light (transmission loss) of an infrared ray having a wavelength within a range of from 2 xcexcm to ten-odd xcexcm is extremely increased as shown in FIGS. 3(a) to 3(c). Because of this reason, applicable wavelength is limited and thus, the light within this range of wavelength is not suitable to spectroscopic analysis in which a variable wavelength laser is used.
On the other hand, it was found out that the above-mentioned absorption peak hardly appears in case calcium fluoride is used as a dielectric. However, its practical use involves the following problems in manufacturing process. That is, calcium fluoride is vapor deposited on an outer periphery of an acid-soluble glass tube (base material) to form a film thereon, then, a silver, for example, is vapor deposited on its outer periphery to form a film thereon and thereafter, the glass tube is dissolved with acid, thereby forming a hollow core portion. At the time of vapor deposition of silver, the silver is dispersed within the calcium fluoride. This makes it difficult to apply plating on an outer periphery of the silver. Moreover, the transmission lose is increased because light absorption occurs in the dielectric layer.
In order to solve the above problems, the present invention relates to a hollow optical fiber having a dielectric layer and a metal layer formed on an outer periphery thereof. Among all, it chiefly relates to a hollow optical fiber, in which a dielectric layer is vapor deposited on an outer periphery of an acid-soluble base material to form a film thereon, then, a metal layer is vapor deposited on an outer periphery of the dielectric layer to form a film thereon and thereafter, the base material is dissolved with acid, thereby forming a hollow core portion. According to the present invention, in such a hollow optical fiber as just mentioned above, the dielectric layer comprises an inner periphery side dielectric layer facing the hollow core portion and an intermediate dielectric layer interposed between the inner periphery side dielectric layer and the metal layer.
The inner periphery side dielectric layer is preferably formed of substance capable of reducing the transmission loss over a rather wide range of wavelength. More preferably, the material is capable of forming a film by vapor deposition and is hardly dissolved with acid. Since calcium fluoride satisfies the above conditions, it is suited to be used as the inner periphery side dielectric layer.
The intermediate dielectric layer is preferably formed of substance which is close in refractive index to the inner periphery side dielectric layer, which is capable of reducing the transmission loss over a rather wide range of wavelength, and which is good in adhesion with respect to both the inner periphery side dielectric layer and metal layer. Moreover, the substance forming the intermediate dielectric layer preferably has such a property which prevents the above-mentioned metal from dispersing therein. It is more preferable for the substance forming the intermediate dielectric layer that a film can be formed by vapor deposition. Since yttrium fluoride satisfies the above conditions, it is suited to be used as the intermediate dielectric layer. No acid-resistance is required for the intermediate dielectric layer.
The metal can be prevented from dispersing by the intermediate dielectric layer formed of yttrium fluoride, and light absorption can be prevented from occurring within the inner periphery side dielectric layer and intermediate dielectric layer. Only one fiber can cope not only with transmission of plural laser beams having different wavelengths but also with transmission of a laser beam having a variable wavelength. Thus, it can be used as a spectroscopic probe.
To manufacture the above hollow optical fiber, it is preferred that an inner periphery side dielectric layer formed of calcium fluoride or the like is vapor deposited on an outer peripheral surface of a base material of an acid-soluble glass tube or the like to form a film thereon, then yttrium fluoride (intermediate dielectric layer) is vapor deposited on an outer peripheral surface of the inner periphery side dielectric layer to form a film thereon, then a metal layer such as silver, gold or the like is vapor deposited on an outer peripheral surface of the intermediate dielectric layer to form a film thereon, and thereafter, the base material is dissolved with acid, thereby forming a hollow core portion.